The present invention relates to a corrosion-prevention system, and more particularly to a metallic piping system having exterior and interior surfaces which are subjected to the effects of spontaneous corrosion.
Spontaneous corrosion on metallic pipes is a well-recognized problem in the prior art. Metallic inhomogeneities and temperature gradients along the pipe are among several contributing factors which cause one metallic region of the pipe to serve as an anode and another metallic region of the pipe to serve as a cathode. In the presence of liquid, an electrolytic action will ensue. The anodic metallic region will tend to react or "waste" away, thus causing the pipe to corrode and to be destroyed.
One prior art approach has attempted to solve this problem by employing sacrificial anodes. Briefly stated, the sacrificial anode is electrically connected to the metallic pipe surface to be protected and is made of a metal material which is higher in the electromotive series (e.g. zinc, aluminum, magnesium, or alloys of these materials) than the metal material (e.g. iron or other ferrous materials) to be protected. Hence, the sacrificial anode will react or dissolve into the electrolytic medium before the metallic material to be protected.
This approach has the obvious disadvantage that the sacrificial anodes need to be constantly replaced with fresh material. Thus, maintenance of the system is expensive and burdensome.
A further approach is to coat a layer of an anti-corrosion substance on a metallic pipe surface to be protected. This approach has the disadvantage that it is very difficult to apply a continuous coating over the entire length of the pipe, especially on its interior surfaces. Undesired scratches inevitably occur as a result of poor application, handling, etc. Alternatively, the metallic portions are commonly exposed due to the coating being melted as a result of the heat caused by welding two adjacent metallic pipe sections together. In either case, this passive protection fails to protect such exposed portions, and corrosion quickly follows.
Another approach is to use an impressed current system. An inert anode, i.e. one which will not waste or dissolve into the electrolyte medium, is mounted in close proximity to the metallic surface to be protected. A positive source of direct current is electrically connected with the inert anode, and a protective electric field is thereby set up which opposes the electrical field caused by the chemico-physical variations of the pipe.
This approach suffers the drawback that each inert anode must be individually connected to a source of positive current. In other words, in applications where the piping extends over great distances, electrical cable must be supplied over this extended distance. Moreover, the electrical connections must be made at each individual inert anode. This operation is both costly in terms of labor and material.